Medical Physiology, 3rd Edition

CHAPTER 45. Nutrient Digestion and Absorption

Henry J. Binder, Charles M. Mansbach II

In general, the digestive-absorptive processes for most of the constituents of our diet are highly efficient. For example, normal adult intestine absorbs ~95% of dietary lipid. However, we ingest most of the constituents of dietary food in a form that the intestine cannot readily absorb. Multiple digestive processes convert dietary food to a form that can be absorbed—primarily in the small intestine, but also, to a much smaller extent, in the colon.

The digestive process—the enzymatic conversion of complex dietary substances to a form that can be absorbed—is initiated by the sight, smell, and taste of food. Although some digestion (that of carbohydrates) begins in the mouth and additional digestion may occur within the lumen of the stomach, most digestive processes occur in the small intestine. Digestion within the small intestine occurs either in the lumen, mediated by pancreatic enzymes, or at the small-intestinal brush-border membrane (membrane digestion), mediated by brush-border enzymes. Several different patterns of luminal, brush-border, and cytosolic digestion exist (Fig. 45-1). Some of the dietary carbohydrate and protein that escape digestion and absorption in the small intestine are altered in the large intestine by bacterial enzymes to short-chain fatty acids (SCFAs) N45-1 that are absorbed by the colon.

FIGURE 45-1 General mechanisms of digestion and absorption. Digestion-absorption can follow any of five patterns. First, the substance (e.g., glucose) may not require digestion; the intestinal cells may absorb the nutrient as ingested. Second, a polymer (e.g., protein) may be digested in the lumen to its constituent monomers (e.g., amino acids) by pancreatic enzymes prior to absorption. Third, an oligomer (e.g., sucrose) is digested into its constituent monomers (e.g., monosaccharides) by brush-border enzymes prior to absorption. When in free solution, fructose is present primarily as the pyranose (6-membered ring) form and less so as the furanose (5-membered ring) form. Fourth, an oligomer (e.g., oligopeptide) may be directly absorbed by the cell and then broken down into monomers (e.g., amino acids) inside the cell. Finally, a substance (e.g., TAG) may be broken down into its constituent components prior to absorption; the cell may then resynthesize the original molecule.

Chain Length

Contributed by Emile Boulpaep, Walter Boron

The digestive processes for carbohydrates, proteins, and lipids result in the conversion of dietary nutrients to chemical forms for which intestinal absorptive processes exist. As a consequence, the digestive-absorptive processes for the several dietary constituents are closely integrated and regulated biological events that ensure survival. Multiple diseases can alter these digestive-absorptive processes and can thereby impair nutrient assimilation (i.e., the overall process of digestion and absorption). Because of the substantial segmental distribution of nutrient absorption along the gastrointestinal tract (Fig. 45-2), the clinical manifestations of disease (Table 45-1) often reflect these segmental differences.

FIGURE 45-2 Sites of nutrient absorption. A, The entire small intestine absorbs carbohydrates, proteins, and lipids. However, the absorption is greatest in the duodenum, somewhat less in the jejunum, and much less in the ileum. The thickness of the arrows indicates the relative magnitude of total absorption at the indicated site in vivo (see inset). The maximal absorptive capacity of a specific segment under optimized experimental conditions (e.g., substrate concentrations) may be greater. B, Some substances are actively absorbed only in the duodenum. C, Bile acids are absorbed along the entire small intestine, but active absorption occurs only in the ileum. D, The vitamin cobalamin is absorbed only in the ileum.

Carbohydrate Digestion

Carbohydrate Absorption

Protein Digestion

Protein, Peptide, and Amino-Acid Absorption

Lipid Digestion

Lipid Absorption

Digestion and Absorption of Vitamins and Minerals

Nutritional Requirements

References